1. Field of the Invention
The present invention relates to drilling systems and more particularly to precision drilling systems for drilling into circuit boards. In drilling circuit boards, it is often critical to achieve drilling to a predetermined depth with reference to the surface of the circuit board within a very tight tolerance. For example, in the case where a circuit board being drilled is a laminated board having interleaved layers of metal and insulating substrate, it is often necessary to drill "via" holes to a predetermined metal layer. If the drilled depth is inaccurate, the board may be rendered defective. Another example of a requirement for precise drilling is the situation in which a stack of boards is drilled simultaneously. In order to increase the throughput of the drilling apparatus, the boards are stacked to a thickness which is almost equal to the length of the particular drill bit being used. For very small holes, the drill bit is typically formed with a shank of increased diameter for connection to the drilling spindle. If drilling progresses too far such that the shank contacts the surface of the top circuit board being drilled, the drill bit may well be broken and/or the top circuit board damaged. Thus, precision in drilling depth is required both in the situation of blind drilling to a predetermined depth and in the drilling of through holes in which the thickness of the stack of boards being drilled is close to the flute length of the drill bit being used.
2. Description of the Prior Art
Drilling systems presently employed can achieve very accurate drilling from a starting point. A typical drill may have a resolution of one tenth of a thousandth of an inch. Prior art systems typically provide a predetermined stop to limit downward travel of the drill spindle to achieve desired drilling depth.
Although the drilling mechanism itself has a high degree of resolution, the board surface to be drilled does not. For example, one typical multilayer board has a thickness of two hundred thousandths of an inch (0.2 inches) and it is desired to drill holes having a depth of four and one-half thousandths of an inch within a tolerance of one-half thousandth of an inch. Because of the thickness variations of the board, it is difficult to drill with such precision since the drilling is not referenced to the board but rather to the support surface of the board.
Circuit board drilling systems typically include a pressure foot surrounding the drill bit which contacts the circuit board to hold it in place during drilling. One prior art system provides a feedback sensor attached to the pressure foot to provide a position indication of the feedback sensor, and thus board surface, which is compared to a sensor location of the drill bit. The disadvantage of such a system is that it does not allow for surface irregularities, since the pressure foot contacts the board surface at a point spaced from the actual drilling point. In addition to surface irregularities of the board, debris on the board surface can also interfere with proper drilling. Also, inaccuracies of the positioning sensors can cause the devices to operate unreliably.